SO2, NOx, and dust are main pollutants in the atmosphere and are a significant source of smog. With the increasing total industrial pollution load and the limited environmental receptivity, the emission standards need to be further improved, to reduce the concentration of emitted pollutants. FCC tail gas is characterized by large fine particulate level (particulates with a size of 0-10 μm account for 50% or more) and high SO2 concentration (300-4500 mg/m3). Moreover, the dust level fluctuates greatly; particularly when the catalyst loss occurs in a regenerator during the regular “soot blowing” process and in an extreme state of a high-temperature operation, the dust level is increased dramatically. In addition to silicon, aluminum and other metal elements, the dust may also contain nickel, vanadium or other heavy metal elements, thus affecting the quality of by-products, and affecting the recycling of sulfur. All these factors have increased the difficulty in effective management of tail gas pollution occurring during catalyst regeneration of a catalytic cracking unit.
At present, dust removal from and desulfurization of the FCC tail gas may rely on the use of wet washing technology; however, the investment, operation and maintenance costs are high, and the construction period of the system is long. Moreover, these technical methods also have the problems such as high consumption of lye, large volume of waste water and others. The existing sodium process is a disposable process, in which the desulfurization and dust removal operations have no need to be separated; however, high salt waste water is required to be treated, secondary pollution may be caused, and a large amount of catalyst may enter the washing liquid under the operating condition of catalyst loss, which may increase the treatment load of the waste solid, and waste the catalyst, thus affecting the long-term stable operation of the system.
Chinese Patent Application No. CN 104941423A disclosed a system for denitrification and desulfurization of and dust removal from an FCC regenerative tail gas by an ammonia-based process on Sep. 30, 2015. The application discloses introducing the high-temperature tail gas containing the catalyst dust produced during the catalyst regeneration of a catalytic cracking unit to a waste heat recovery boiler I, where the temperature of the tail gas is reduced to 280-430° C., and the heat of the tail gas is utilized by the waste heat recovery boiler I to produce steam for output; entering the tail gas at 280-430° C. into a denitrification system for denitrification; after adequate reaction on the surface of the denitrification catalyst in the denitrification reactor, entering the tail gas into a waste heat recovery boiler II via a tail gas vent; removing the sulfur dioxide and nitrogen oxide in the tail gas by reacting with ammonia, with ammonium sulfate being produced as a by-product, and removing the catalyst dust in the tail gas at the same time, to obtain a cleaned gas that is discharged up to standard.
The application discloses integrated desulfurization and dust removal technology by an ammonia-based process. During the long-term operation and practice, where the same absorption liquid is used for dust removal and desulfurization, the absorption liquid containing ammonium sulfate is difficult to separate from the dust because the particle size of the catalyst dust is small. The system necessitates that the dust level in the tail gas at the inlet is 30-800 mg/Nm3. The dust removal and the desulfurization are carried out at the same time, and there is mutual interference, thus affecting the long-term stable operation of the apparatus. Especially when the dust level in the tail gas at the inlet is as high as 5000 mg/Nm3, and the total amount exceeds 2 tons under an accident condition, ingredients entering the circulating absorption liquid will affect the absorption liquid, and the desulfurization and dust removal efficiencies, and the absorption liquid cannot be effectively separated from the dust, thus affecting the quality of the product ammonium sulfate.
It would therefore be desirable to provide improved apparatus and methods for recovery-type deep denitrification, desulfurization and dust removal to overcome shortcomings in the prior art.